Biology of Grass - The Accidental Smallholder

I’m including a short section on grass biology; I had to cover this as part of my agricultural degree. I have to confess, I find it more interesting now than then, but I’m surprised at how much I remember. And at how much I’ve forgotten. Understanding the biology of grass helps us understand how to manipulate our grassland to suit our aims and objectives.

Grass grows almost everywhere on Earth, apart from the most extreme desert. There are more than 10,000 known species of grass; it is one of the largest families of plants and probably the most important in terms of use. Many of the crops we grow – wheat, barley, oats, rice, maize, sugar cane – are grasses that have been selected to produce heavy seed crops. There are grass species that are adapted to grow in the most inhospitable places.

Angiosperms

Grass is an angiosperm, which means it is a flowering plant, although the flowers are small, green and inconspicuous. Grass flowers are wind pollinated and the seeds are dispersed by wind, water, by birds and on the coats and sometimes in the digestive systems of animals. However, grass can also spread by tillering; this means it can spread laterally by producing side shoots over ground or roots underground that produce new plants. This is why grass can persist even if it is cut or grazed and never gets the chance to set seed.

The angiosperms are divided into two groups; monocotyledons, which produce seedlings with a single leaf, such as grasses and dicotyledons, which produce seedlings with two leaves.

Grasses can be annual, biennial or perennial and this governs their growth habit and persistency. Different species flower at different times of the year, with the majority of British grasses flowering in May, June and July. Preventing flowering by removing the grass stem, by grazing or cutting, causes the plant to reproduce vegetatively by tillering; this produces a leafier, denser sward than one that is allowed to flower and set seed.

Nutritional value of grass

Most of the nutritional value of grass is in the leaf, and grass is at its most nutritious and with the highest moisture content early in the growing season. However, total yield and % dry matter increases over the season, until the plants die back in winter.

Particularly when cutting for conservation, a balance needs to be struck between nutritional value and yield – cutting early will give low yields of highly nutritious fodder with a low dry matter content; cutting late, when much of the grass has gone to seed, will give high yields of poorer quality fodder with high dry matter. Allowing the grass to seed also adversely affects the density of the sward, and therefore its ability to withstand adverse conditions.

Constitution of grassland

Of course, grassland isn’t just grass and almost certainly isn’t just one species of grass, except in the case of some short-term leys. Short-term grass leys usually form part of a crop rotation, are regularly ploughed and reseeded and may consist of just one or two varieties of grass, usually perennial and / or Italian ryegrass. They may also contain clover if it has been sown too but they tend to be quite intensively managed and high nitrogen input doesn’t suit clover.

The use of broad-leafed herbicides (weedkillers which kill plants with broad leaves but don’t affect narrow leaved grasses) will reduce the number and type of other plants, including clover, and high inputs of nitrogen favour the ryegrasses, which will outcompete other species of grass and other plants.

Grass grows most rapidly where the average rainfall is between 500 and 1500mm per year. The soil type and the height of the water table also influences how much water is available to the grass plants; a lack of water in the ground will also adversely affect the amount of nitrogen and other nutrients available to the plant.

Growing season of grass

The length of the growing season of grass is governed by temperature. In conditions of less than 5°C, grass is virtually dormant. Grass starts to grow rapidly in spring as soil temperatures rise and there is plenty of moisture in the soil.

Growth slows in summer when there is less moisture but there is often a second flush in September as rainfall increases and soil temperatures remain high. Applying fertiliser when the grass is not actively growing is both a waste of money and detrimental to the environment – because the grass cannot use the fertilizer, it will be washed out of the soil and into watercourses if there is heavy rain.